The endothelium regulates the contractile state of cerebral arteries and arterioles through the release of relaxing factors. In recent years, an endothelial-dependent dilator process, other than nitric oxide (NO) or prostacyclin, has been discovered. This new process, termed "endothelium-derived hyperpolarizing factor" or EDHF, is upregulated to compensate for diminished endothelial-derived NO following traumatic brain injury. Thus, EDHF could be an important mechanism to maintain cerebral perfusion when the NO dilator mechanism is compromised. This proposal addresses the mechanism of the EDHF-mediated dilations in cerebral arteries following traumatic brain injury to the rat. In Specific Aim 1, studies are proposed to determine if the mechanism of EDHF-mediated dilations following traumatic brain injury involves the metabolism of arachidonic acid through the P450 epoxygenase pathway. In Specific Aim 2, studies are proposed to determine if the mechanism of EDHF-mediated dilations following traumatic brain injury involves an increased production of hydrogen peroxide (H2O2). In Specific Aim 3 studies are proposed to measure Ca2+ and membrane potential in endothelium and vascular smooth muscle during EDHF dilations following traumatic brain injury. Diameter changes of pressurized branches of middle cerebral arteries (bMCAs), isolated from injured (controlled cortical impact injury) and non-injured cortex, will be compared following agonist induced EDHF dilations. A combination of pharmacological interventions, optical methods for selectively measuring [Ca2+] and membrane potential of vascular smooth muscle and endothelium, electrophysiological techniques, and analytic methods to measure P450 epoxygenase metabolites and H2O2 are proposed to address the aims. In Specific Aim 4 EDHF dilations will be studied in vivo during normal conditions and following traumatic brain injury. We propose to study the EDHF response using laser Doppler flowmetry and measurement of pial arteriole diameter. We speculate that EDHF upregulation is an important intrinsic mechanism to maintain cerebral perfusion. Understanding the mechanism of EDHF-mediated dilations following traumatic brain injury will allow better insight into the regulation of cerebral blood flow following brain injury. Further, it will allow us to test the hypothesis that the EDHF upregulation is protective and could lead to new therapeutic strategies for treatment of traumatic brain injury.